Have you read Pat Morris’ book ‘Hedgehogs’? Because in there is a chapter entitled ‘Rehabilitating Hedgehogs – Does it work?’ and ‘Can Inexperienced animals cope with release into the wild?’ It’s a while since I read it, and it may not have been in those chapters, but I recall that some released hogs in a study didn’t make it – thought to be (in part) because they didn’t have appropriate responses to predators. The reaction of the hog there lying on his back brought it to mind. It may be that they learn some ‘unusual’ habits in captivity and it takes them a while to learn to be wild again.
Maybe the results of the survey they did recently regarding rehabilitating, etc. will throw up some more useful information.